1. Field of the Invention
This invention relates to a multi-slit type spectrometer.
2. Description of Prior Art
A lot of attentions have been conventionally given to a non-destructive inspection by use of near infrared spectroscopic analysis. Particularly, there has been a strong demand for an on-line non-destructive inspection in manufacturing industrial products. However, a near infrared spectrometer which is presently in practical use requires a long time for the inspection since a diffraction means such as a diffraction grating is mechanically scanned. This spectrometer is unsuitable to carry out a continuous inspection over a long time since it has a mechanical drive device. In view of this, recently, a multi-slit spectroscopic method has been employed which is suitable to carry out the inspection within a short time has been employed. In order to eliminate the need for the drive device, also, spectrometers have been proposed which have an optical shutter including a multitude of electrically controllable slits.
U.S. Pat. No. 4193691 discloses a spectrometer employing a liquid crystal shutter as an optical shutter. In this spectrometer, an incident light is separated into respective wavelength components by diffraction means and a plurality of liquid crystal shutters are arrayed on an imaging plane of a spectrum. The respective wavelength components are modulated with different frequencies by turning on and off the respective liquid crystal shutters. The modulated rays are again concentrated to one beam, which is detected by a detector. An output signal of the detector is demodulated with the same frequencies as the modulation frequencies of the respective wavelength components, and the intensity of each wavelength component is detected.
Further, spectrometers have been proposed which employ an optical shutter array member formed of dichromic material (U.S. Pat. No. 4615619, U.S. Pat. No. 4799795, Japanese Examined Patent Publication No. 5-6857). The dichromic material has a property of changing from a transparent state to a light reflecting state when the temperature rises. This property of the dichromic material is used to function as an optical shutter. The temperature of the dichromic material is controlled by turning on and off a current applied to this material.
However, the above liquid crystal shutter has a slow responsiveness because of the property of liquid crystal, and cannot be turned on and off at a high speed. It is also relatively difficult to turn on and off the optical shutter made of dichromic material at a high speed because there is a time delay between an actual temperature rise and the start of application of the current. In addition, it is not easy to control the temperature of the dichromic material.